1. Field of the Invention
The present invention relates to dielectric filters and dielectric duplexers, and more particularly, to a dielectric filter for use in a communication unit in the microwave band and the millimeter-wave band.
2. Description of the Related Art
A conventional type of dielectric filter for use in a communication unit in the microwave band and the millimeter-wave band is a multiple-stage filter in which dielectrics having a low dielectric constant sandwich a plurality of TEM-mode coaxial resonators (see Japanese Unexamined Patent Publication No. 2-94903). In this type of dielectric filter, a TEM-mode coaxial resonator in each stage independently functions as a resonator.
Communication units using such a filter have been required to handle higher-frequency signals as more channels have been demanded, but at a high frequency (such as in the 3-GHz band), a dielectric filter formed of a plurality of TEM-mode coaxial resonators has a very low no-load Q value and high transmission losses.
To overcome the above problems, there has been proposed a waveguide-type dielectric filter 40 using the TE.sub.10 mode, as shown in FIG. 11 for example. This dielectric filter 40 has three TE.sub.10 -mode resonators 51 and two dielectric coupling windows 52. The TE.sub.10 -mode resonators 51 are connected in series with the dielectric coupling windows 52 disposed therebetween.
The resonators 51 and the dielectric coupling windows 52 are formed of a dielectric block 41 made from one material and having almost a rectangular-parallelepiped shape, on which an outer conductive member 43 is provided to cover almost the entire surface of the dielectric block 41. A pair of input and output electrodes 45 not electrically connected to the outer conductive member 43 with the a gap between the electrodes 45 and the member 43 are provided respectively at the two ends of the dielectric block 41. To set the central frequency of each resonator 51 to the desired value, the length of each resonator 51 needs to be set to approximately half the wavelength .lambda. of the central-frequency signal. To set the pass-band width of the dielectric filter 40 to the desired value, it is necessary to set the width W1 of the dielectric coupling windows 52 appropriately.
Although the proposed dielectric filter of FIG. 11 has a high no-load Q value and a small loss at a high frequency band, it is difficult to manufacture. Since the resonators 51 and the dielectric coupling windows 52 are made from the same dielectric material in the proposed dielectric filter, they have the same dielectric constant. Therefore, in order to manufacture a plurality of dielectric filters having the same central frequency and different pass-band widths, the width W1 of the dielectric coupling windows 52 needs to be changed and thereby the shape or the dimensions of the dielectric block 41 must be changed. A forming metal die is required for each of the plurality of dielectric filters.
There is a method for manufacturing a plurality of dielectric filters having different pass-band widths with the same forming metal die. A rectangular-parallelepiped dielectric block is formed by a metal forming die. Two pairs of grooves opposing each other with a gap therebetween are formed on both sides of the dielectric block by cutting with a dicing saw. A portion sandwiched between each pair of grooves in the dielectric block serves as a dielectric coupling window. To manufacture a plurality of dielectric filters having the same central frequency and different pass-band widths, it is also necessary in this method to change the blade-feeding distance (which equals the depth of the groove) of the dicing saw to modify the width of the dielectric coupling window for each of the plurality of dielectric filters.